I guess it might depend on where exactly you want to cut them off. My first thought would be make a very rough first incision a long way down the can (maybe using a chisel) - then use tinsnips to cut in a helical fashion gradually up to the area you want. Starting off down at the other end of the can means any brutality involved in making the first incision is less likely to damage the bit you want to end up with.
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http://www.cigarboxnation.com/photo/tri-cone-guitar-17?context=user
It`s been done already...check out Bairfoot Cajuns page. He shows a a bunch of pic`s how he did the cone [ cut soda cans ] set up.
Perhaps with a dremmel?
I guess it might depend on where exactly you want to cut them off. My first thought would be make a very rough first incision a long way down the can (maybe using a chisel) - then use tinsnips to cut in a helical fashion gradually up to the area you want. Starting off down at the other end of the can means any brutality involved in making the first incision is less likely to damage the bit you want to end up with.